A leadframe-based semiconductor device package is the most widely used integrated circuit (IC) package. The leadframe typically includes a chip mount pad (also referred to as a die paddle) for attaching the IC die or chip to the leadframe, and a plurality of lead fingers or conductive segments which provide a conductive path between the chip and external circuits. A gap between the inner end of the lead fingers and the chip is typically bridged with bond wires attached to bond pads on the chip and to the inner end of the lead fingers. Bond wires can be formed from various materials, such as gold, copper, aluminum, or an alloy thereof. The outer ends of the lead fingers remote from the IC chip can be electrically and mechanically connected to external circuitry. After assembly of the chip to the leadframe, the chip, the bond wires, and a portion of the leadframe can be encapsulated in mold compound.
As the encapsulated semiconductor device undergoes temperature cycling during device testing or use, it is known that thermomechanical stresses are induced at the joints or interfaces between dissimilar materials used in the fabrication of the device. The stresses are primarily induced due to differences between the coefficients of thermal expansion (CTE) of the various materials. For example, metal used to fabricate the leadframe assembly expands or contracts differently than plastic resin material used as the mold compound, thereby causing delamination of the mold compound from the leadframe. These stresses, which may be repeatedly induced during hundreds or thousands of temperature cycles, tend to fatigue the joints and the interfaces, and can result in separation between the leadframe and the mold compound. If delamination occurs, the device can fail as a result of physical damage during separation or from corrosion resulting from the intrusion of moisture into the delaminated package.
Various strategies are used to reduce delamination of the mold compound from the leadframe. One technique comprises optimizing the formulation of the mold compound to provide increased adhesion to the leadframe surface. Another technique comprises roughening the leadframe surface using chemical or mechanical processes. Yet another method comprises the application of a liquid adhesion promoter to the leadframe, conventionally using a stencil mask to avoid applying the liquid to the wire bond areas of the inner lead fingers and to the outer lead fingers which will be external to the encapsulation material.